The present invention relates to a method and system for controlling the transmission power of network nodes in a data communications network.
More particularly, the present invention relates to a method and system for controlling network node transmit power levels which can be integrated with a method or system for compensating for signal propagation delay, and which is of particular use in a data communications network where accurate power control and time synchronisation is required.
Data networks can be classified in many ways, but for the purpose of the present invention, it is useful to classify them by their means of accessing the medium over which data is communicated. The relevant classifications are broadcast and non-broadcast
An existing type of data network is Ethernet. Ethernet uses broadcast medium access. All network nodes sharing the network medium hear all traffic being passed over the medium. Traffic is directed to individual network nodes via physical layer addresses that are attached to the data packets being sent over the medium. When multiple network nodes attempt to transmit data simultaneously, there is the possibility for contention among the nodes for access to the medium.
A modification to the broadcast network is the broadcast network with hidden terminals. In this network, all terminals share the same medium, however it cannot be guaranteed that all terminals can hear each other. All that can be guaranteed is that all terminals can hear the central network node, referred to herein as the access point. For this reason, it is not enough for each terminal simply to monitor the channel in order to detect contentions. Feedback on success or failure of network contention must also be communicated back to the network terminals by the access point.
In contrast to the above, in a non-broadcast network, the medium that connects a network node to the rest of the network can only be accessed by two devices: the network node itself, and the network switch to which it is attached. The medium itself is fill duplex, so there is no possibility for contention.
A variation on the contention broadcast network is the time slotted network. In such a network fixed time slots are assigned to all nodes of the network, and the transmissions of each node are restricted to its particular assigned time slot. An example of such an arrangement of the prior art would be a Time Division Multiple Access (TDMA) network.
Such networks are often deployed in cellular configurations. In such configurations, each cell consists of a central access point and multiple subscriber terminals, and subscriber terminals communicate only to the central access point, making the network a point-multipoint architecture. A problem can arise due to the fact that as terminals may be located anywhere within the coverage area of the cell, then if each subscriber terminal transmitted with a fixed transmit power, the signals from each subscriber terminal would be received at the access point with varying power levels, due to the different free space path loss (FSPL) each signal would encounter. In order to overcome this problem, the access point receiver can use Automatic Gain Control (AGC) and a wide dynamic range receiver to receive each burst. However, this requires a long period of energy at the front of each upstream burst, used strictly for AGC loop stabilization therefore reducing channel efficiency. An alternative power control scheme is to use the access point receiver to measure the received signal strength from each subscriber terminal, then send a power control message to each network terminal to increase or decrease its power. However, this alternative scheme suffers from closed loop dynamics, plus reduces network bandwidth efficiency slightly. It also suffers from startup power control loop transients, since the first time the subscriber terminal transmits, it has not yet received any power control information.
The method and system of open loop power control of the present invention improves upon the above described schemes by using the following principle. In the present invention, the power level of the downstream burst arriving from the access point is measured and used to set the upstream transmit power of the network subscriber terminal before the subscriber terminal has ever first transmitted. The present invention is therefore open-loop, and does not suffer from closed loop dynamics.
According to the present invention, there is provided a method of transmission power control for use in a data communications network comprising a central control node and at least one remote subscriber node, said method comprising the steps of:
a) transmitting a first signal from the central control node to the remote subscriber node;
b) measuring a received power level of the first signal received at the remote subscriber node; and
c) setting a transmit power level of the remote subscriber node in response to the received power level;
wherein the transmit power level of the remote subscriber node is set before the remote subscriber node has transmitted any signals onto the network, whereby said method is an open-loop method.
The method steps may be continuously repeated in order, allowing for the transmit power level of the subscriber node to be continuously controlled.
The first data portion may further contain a known data sequence, and this may be used to measure the received signal power.
The setting step may further contain the steps of using the received power level as an index into a look-up table of transmit power attenuator values; selecting the indexed attenuation value; and attenuating the transmit power level by the selected attenuation value.
All data traffic on the network may be regulated per unit time frame by the central control node.
The method of the present invention may be used in combination with any one of or a combination of a method of compensating for baseband delay or a method of compensating for signal propagation delay between network nodes.
From another aspect, the present invention further provides a system for controlling transmit power levels for use in a data communications network comprising a central control node and at least one remote subscriber node, said system comprising:
a) transmission means for transmitting a first signal from said central control node to said remote subscriber node;
b) measurement means for measuring a received power level of the first signal received at the remote subscriber node; and
c) setting means for setting a transmit power level of the remote subscriber node in response to the received power level;
wherein the setting means set the transmit power level of the remote subscriber node before the remote subscriber node has transmitted any signal onto the network, whereby said system is an open-loop system.
Each of the system means may repeat their respective operations in order, allowing the transmit power level of the subscriber node to be continuously controlled.
The first signal may contain a known data sequence, and the signal power of the known data sequence is measured.
The setting means may further comprise: indexing means for using the received power level as an index into a look-up table of transmit power attenuator values; selection means for selecting the indexed power attenuator value from the look-up table; and attenuation means for attenuating the transmit power level by the selected attenuation value.
Any data traffic on the network may be regulated per unit time frame by the the central control node.
The system of the present invention may be used in combination with any one of or a combination of a system for compensating baseband delay or a system for compensating for signal propagation delay between network nodes.
It is an advantage of the present invention in that because it is not closed loop, the control method is instantly stable. Moreover, because the subscriber terminal measures the received signal power of a downstream burst before ever transmitting, there are no initial transients in the power control scheme. The first time the subscriber terminal transmits, it does so at the proper power level.
It is another advantage that the present invention measures the received signal power of a signal which would in any case be required for network signalling, thereby allowing power control to be achieved without adversely affecting bandwidth efficiency.
It is a further advantage that because the subscriber terminal measures the received power on each downstream burst, it is capable of tracking rapid changes in signal propagation.
In addition to the above, there is another advantage that the access point""s tolerance of adjacent channel interferers is improved, since A/D dynamic range in the receiver is not spent on accommodating wide variations in received signal power.
Furthermore, since the network terminal dynamically tracks the signal propagation conditions, transmit power of the network terminal can be maintained at the minimum necessary for acceptable error rates at the access point. This enables efficient power usage at the network terminals, which enables battery powered terminals, and maximises battery life.
Finally, it is a feature of the present invention that power estimation may be done quickly enough to track dynamic fading channel conditions.